The present invention relates to an echo/noise canceler for removing echo and noise components from an input signal.
Echo cancelers and noise cancelers are used in voice communication terminals such as videoconferencing terminals, automobile-mounted hands-free telephone sets, and portable telephone sets. Unexamined Japanese Patent Application No. 8789/1996 describes a voice communication system in which a noise canceler removes noise components from the output of an echo canceler, and the output of the noise canceler is used in adjusting adaptive filter coefficients in the echo canceler. The reason for this arrangement is that use of the output of the noise canceler in adjusting the adaptive filter coefficients enables the residual echo to be reduced to a lower level. In a variation of this arrangement, the output of the echo canceler, which is the input of the noise canceler, is used to adjust the adaptive filter coefficients until the residual echo has been reduced to the local background noise level, then the output of the noise canceler is used to achieve a further reduction.
A problem in this arrangement is the processing delay of the noise canceler. Most recent noise cancelers divide the input signal into frames and process one frame at a time. Noise cancelers employing spectral subtraction, for example, operate in this way. Spectral subtraction and other frame-based noise cancellation methods have the advantage of high accuracy, but they generate an unavoidable processing delay equal to or greater than the frame length.
In the arrangement described above, a long processing delay in the noise canceler can prevent the adaptive filter coefficients in the echo canceler from converging, or can cause the coefficient values to oscillate. Further details will be given below.